In a network configured with a plurality of switching systems each having a packet transfer function, there is a known technology that switching a packet having a label is controlled based on a packet forwarding table provided in each switching system.
The forwarding table includes either an input label or a combination of input port and input label each corresponding to a combination of output port and output label.
In the conventional method, a switching system performs packet transfer when a short and fixed-length label included in each packet coincides completely with any of the contents of such a forwarding table. This enables high speed transfer processing. Also, because each communication is mainly performed through an established connection, the method is also suitable for traffic engineering such as load sharing.
In regard to such a label switching method, standardization has been studied and developed in Multi Protocol Label Switching (MPLS) Working Group in the Internet Engineering Task Force (IETF).
In the meantime, an optical wavelength division multiplexing (WDM) network begins to use to cope with traffic increase. In such a WDM network, a wavelength switching system has been attracting a wide attention, in which wavelength switching system is combined with WDM transmission equipment.
Such a wavelength switching system can be regarded equivalent to a label switching system by regarding a wavelength as a label. As a procedure of modifying a path route in such either label switching system or wavelength switching system, a route modification procedure specified in Constraint-based Routing Label Distribution Protocol (CR-LDP) is applicable.
However, there is a problem in this route modification procedure specified in this CR-LDP. On the way to the route modification, despite that information actually flows on only one route at a time, two labels for both a new route and an old route are required to reserve in a label switch router (LSR) being located where these old and new routes overlap.
In other words, on such location where the old route and new route overlap, it is not possible in the conventional method to share a single label for both old and new routes. This results in waste of labels though it occurs temporarily during the modification procedure. Therefore, this route modification procedure specified in CR-LDP becomes ineffective in such a situation as the reservation of an extra label is not possible.
Generally in a label switching system, there is sufficient label space and no relation between labels and other resources. Therefore, the aforementioned problem will not cause a serious problem. However, in an optical wavelength switching system in which an optical wavelength is regarded as a label, available wavelengths are closely related to the system bandwidths, and regrettably the number of multiplexed wavelengths in an optical fiber is not sufficiently large at present. This results in a limited label space in an optical wavelength switching system and may cause a problem, which cannot be overlooked.